Glucocorticoids such as prednisone, Dexamethasone (DEX), and budesonide are the most effective anti-inflammatory drugs. They are widely used to treat inflammation and autoimmune diseases such as asthma, arthritis, lupus, and Crohn's disease (1, 2). These drugs exert their physiologic roles through binding to the glucocorticoid receptor (GR), a ligand-activated transcriptional factor of the nuclear receptor superfamily. In the absence of glucocorticoid, GR resides in the cytoplasm and associates with chaperone proteins such as hsp90 and hsp70. The binding of hormone causes a conformational change in GR, leading to its translocation to the nucleus, where it exerts its transcriptional control activity, either activation (transactivation) or repression (transrepression). In transactivation, GR dimerizes, binds directly to a specific glucocorticoid response element, and then recruits coactivators to activate transcription. In transrepression, the general model is that GR binds other transcription factors (e.g., NF-κB, AP-1) to become indirectly tethered to their binding sites through protein-protein interactions. Upon being tethered near a target promoter, GR represses downstream gene expression (3). It is generally believed that transrepression does not require GR dimerization (4, 5).
Transrepression is the major mechanism through which glucocorticoids act as anti-inflammatory agents (6). The tethering of GR to the NF-κB/AP-1 promoter leads to the transcriptional repression of major downstream proinflammatory factors, including proinflammatory cytokines (e.g., TNF-α, IL-1β, and IL-6), chemokines (e.g., CCL2, CCL19), and enzymes associated with the onset of inflammation (e.g., COX2, MMP13, and phospholipase A2) (2). Because of their fast action and sustainable effect, glucocorticoids remain the first choice for treating inflammatory diseases. However, the long-term use of glucocorticoids, especially at high doses, has many adverse consequences, including diabetes mellitus/glucose intolerance, hypertension, obesity, and osteoporosis (7, 8). Most of these consequences are attributed to the transactivation of GR. For instance, glucocorticoids induce the genes encoding rate-limiting enzymes of the gluconeogenesis pathway in liver, glucose-6-phosphatase and phosphoenol pyruvate carboxykinase (9, 10), thus augmenting the de novo synthesis of glucose and eventually leading to weight gain or diabetes. Glucocorticoids also induce a key regulatory gene of bone development, Dickkopf-1 (DKK1), up-regulation of which leads to osteoporosis and bone loss (11). It is generally observed that many of the side effects of glucocorticoids are associated with high dose usage of glucocorticoids (12-14). For example, a “threshold pattern” was observed for the use of prednisone: at 7.5 mg per day, it causes glaucoma, depression and high blood pressure (12). These side effects are caused by GR transactivation as well as by non-target activation of other receptors such as mineralocorticoid receptor (MR), which activation cause high blood pressure (15). Thus, it is important to develop highly potent and selective glucocorticoids to reduce the unwanted side effects.
Potency and efficacy are two key pharmacokinetic parameters of glucocorticoids. While efficacy is the maximal activity a given drug can achieve, usually at maximal concentration, potency is the concentration of a given drug required to reach half maximal activity (EC50). For two glucocorticoids that have the same efficacy, a highly potent one will require a lower dose to achieve the same treatment effect (14, 15). Importantly, a glucocorticoid may have different potencies for transactivation and transrepression; for example, gene induction by GR via DEX requires a 5- to 6-fold higher glucocorticoid concentration than gene repression (16-18). This differential response provides an opportunity to develop highly potent glucocorticoids that can be used at low doses to achieve full repression of inflammation signals while with minimal transactivation activity and side effects. Finally, the development of insensitivity and resistance to glucocorticoid therapy is a major problem in treating common inflammatory diseases such as chronic obstructive pulmonary disease, rheumatoid arthritis and inflammatory bowel disease (19). Glucocorticoid resistance is also an unsolved issue for white blood cell cancers, especially childhood acute leukemia (20). Several mechanisms of glucocorticoid resistance have been identified or proposed, including a change of kinase pathways, alteration of cofactors, and loss or mutation of receptors (19, 21). One common observation is that the affinity of ligand for receptor is decreased in glucocorticoid-resistant patients. Such patients treated with highly potent glucocorticoids have shown improvement, but the effect gradually decreased (22). Therefore, there is an urgent need to develop a new generation of more potent glucocorticoids.
Cortisol is an endogenous glucocorticoid produced by the adrenal gland. Cortisol has low potency and receptor binding ability relative to the most commonly used synthetic glucocorticoid, such as DEX (23). On the other side, Mometasone Furoate (MF) is a potent glucocorticoid used to treat inflammatory skin disorder (Elocon), asthma (Asmanex), and nasal sinus inflammation (Nasonex) (24, 25). MF has a lipophilic furoate ester at the C17α position of steroid D ring, which is believed to be the origin of its high potency (26). Here the inventors determined the crystal structures of the GR LBD bound to MF and cortisol, which reveal the underlying mechanism that discriminates the ligand potency between MF and cortisol. We then used the observed structure mechanism to design several novel glucocorticoids with much improved potency and efficacy, which could serve as the starting leads for therapeutic development for treating inflammatory diseases.